In a communication network for transmitting and receiving email, and in a mobile telephone network, for example, it is necessary to provide an uninterrupted service to users. To this end, there may be provided in such networks devices that duplicate the function of corresponding devices, and which are employed when a corresponding device fails or is required to be serviced. Such “duplicate” devices provided in a communication network consist of a pair of devices having an identical or almost identical function(s). When one of the pair of duplicate devices is in an operational state, the other duplicate device of the pair is in a standby state. In a case that the device in an operational state malfunctions or is the subject of a maintenance operation, the device in the standby state is made operational. Hereinafter, to differentiate such duplicate devices, a device in an operational state will be referred to as a “primary” device, and a device in a standby state will be referred to as an “auxiliary” device.
Various techniques have been proposed for use in a communication system such as a communication network for transmitting and receiving email in which primary and auxiliary devices are employed so as to avoid suspension of service. For example, Japanese Patent Application Laid-open Publications 2001-168899, H10-207802, and H04-278732 each disclose a technique for enabling short-time switching between a primary device and an auxiliary device (hereinafter, JP 2001-168899, JP H10-207802, JP H04-278732, respectively); and Japanese Patent Application Laid-open Publications 2000-250771, H11-261663, and H08-023334 each disclose a technique for switching between a primary device and an auxiliary device while avoiding interruption of service (hereinafter, JP 2000-250771, JP H11-261663, JP H08-023334, respectively).
In the technique disclosed in JP 2001-168899, JP H10-207802, and JP H04-278732, although a time required for switching between a primary device and an auxiliary device is kept short, it is not completely eliminated. Thus, for a certain amount of time during a switching period, processes in the communication network are unable to be carried out by either the primary or auxiliary device. In a case of a communication network which is an in-house LAN, the scale of the network is likely to be relatively small, and a volume of exchanged email is likely to be relatively small. However, in a case of a communication network operated by a carrier, for example, where a volume of exchanged email may amount to several hundred to several thousand mails per second, a time required for switching between a primary and auxiliary device may be as long as 2-3 minutes. This switching time is problematic in that it impedes functioning of a mail server of the network. This is particularly the case where, for example, a primary mail server which writes and reads email to/from a storage device is switched to an auxiliary device having the same function. In such a case, a large number of email for forwarding to the mail server will accumulate in a mail reception server provided upstream of the primary mail server and processing of this mail will therefore be delayed. There is also a danger that the mail reception server will be overloaded by the accumulation of mail, and consequently that during a switching period the mail reception server may be no longer able to receive any additional mail. A further problem exists in that to enable switching between a primary and auxiliary mail server, it is necessary for there to be kept available in the mail server a certain minimum amount of free memory.
The techniques disclosed in JP 2000-250771, JP H11-261663, JP H08-023334 enable switching between a primary device and an auxiliary device instantaneously without interruption. However, these techniques require that processes carried out by a primary device be constantly updated to an auxiliary device. Thus, it is necessary to frequently synchronize data between a primary device and an auxiliary device, which requires use of a communication system having a complicated configuration, and constant operation of the auxiliary device, which results in high operational costs.
The present invention has been envisaged in view of the problems described above, and has as its object a technique for switching between a primary and auxiliary device in such a way that avoids any interruption in a communication service, and which can be achieved by way of a simple configuration.
In one aspect, the present invention provides a communication system having two write devices for writing data into a storage device, wherein when one of the write devices is in an operational state the other of the write devices is in a standby state, and when data is received, the one of the write devices in the operational state writes the received data into the storage device, the communication system comprising: a transfer device; and a data save device, wherein the transfer device comprises: first receiving means for receiving data for the communication system; and transfer means for transferring to the data save device data received by the first receiving means during a switching period from commencement to completion of an operation for switching from one of the write devices in an operational state to the other in a standby state, the data being transferred to the write device in the operational state outside the write device switching period, wherein the data save device comprises: second receiving means for receiving data transferred from the transfer device; storage means for storing the data received by the second receiving means; and transmission means for transmitting, when the write device switching period is completed, the data stored in the storage means to the write device in the operational state.
According to the present invention, data transmitted to the communication system during a write device switching period is stored by the data save device, and the data is transmitted after completion of the write device switching period to a write device in an operational state, to be stored in the storage device.
In another aspect, the present invention provides a communication system having two management devices for managing data stored in a storage device, wherein when one of the management devices is in an operational state the other is in a standby state, and when process definition information defining details of a process to be performed for the data is received, the one of the management devices in the operational state executes the process according to the process definition information and then outputs a result of the process, the communication system comprising: a transfer device; and a data save device, wherein the transfer device comprises: first receiving means for receiving process definition information for the communication system; and transfer means for transferring the process definition information received by the first receiving means to the data save device during a switching period which lasts from commencement to completion of an operation for switching from one of the management devices in an operational state to the other of management devices in a standby state, the process definition information being transferred to the management device in the operational state outside the switching period, wherein the data save device comprises: second receiving means for receiving the process definition information transferred from the transfer device; storage means for storing the process definition information received by the second receiving means; and transmission means for transmitting, when the management device switching period is completed, the process definition information stored in the storage means to the management device in the operational state.
According to the present invention, process definition information transmitted to the communication system during a management device switching period is stored by the data save device, and after completion of the management device switching period, the data is transmitted to a management device in an operational state, to be processed.
In still another aspect, the present invention provides a communication system having a plurality of data write units, with each unit comprising two write devices for writing data into a storage device, wherein when one of the write devices is in an operational state the other is in a standby state, and when data is received, the one of the write devices in the operational state writes the received data into the storage device, the communication system comprising: a transfer device; a data save device; and detection means for detecting a data write unit that has started an operation for switching between the write device in the operational state and the write device in a standby state, wherein the transfer device comprises: first receiving means for receiving data for the data write unit detected by the detection means; and transfer means for transferring the data received by the first receiving means to the data save device during a switching period from commencement to completion of an operation for switching from one of the write devices in an operational state to the other in a standby state, the data being transferred to the write device in the operational state in the data write unit detected by the detection means outside the switching period, wherein the data save device comprises: second receiving means for receiving data transferred from the transfer device; storage means for storing the data received by the second receiving means; and transmission means for transmitting, when the write device switching period is completed, data stored in the storage means to the write device in the operational state in the data write unit detected by the detection means.
Further, the present invention provides a communication system having two mail servers, wherein when one of the mail servers is in an operational state the other is in a standby state, and when an email is received, the one of the mail servers in the operational state writes the received email into a storage device, the communication system comprising: a transfer device; and a data save device, wherein the transfer device comprises: first receiving means for receiving an email at the communication system; transfer means for transferring an email received by the first receiving means to the data save device during a period from commencement to completion of an operation for switching from one of the mail servers in an operational state to the other in a standby state, the email being transferred to the mail server that is in the operational state outside the mail server switching period, wherein the data save device comprises: second receiving means for receiving an email transferred from the transfer device; storage means for storing the email received by the second receiving means; and transmission means for transmitting, when the mail server switching period is completed, the email stored in the storage means to the mail server in the operational state.
Still further, the present invention provides a communication system having two mail servers, wherein when one of the mail servers is in an operational state the other is in a standby state, and when process definition information defining details of a process to be performed for an email is received, the one of the mail servers in the operational state executes a process according to the received process definition information and outputs a result of the process, the communication system comprising: a transfer device; and a data save device, wherein the transfer device comprises: first receiving means for receiving the process definition information for the communication system; transfer means for transferring the process definition information received by the first receiving means to the data save device during a period from commencement to completion of an operation for switching from one of the mail servers in an operational state to the other in a standby state, the process definition information being transferred to the mail server in the operational state outside the mail server switching period, wherein the data save device comprises: second receiving means for receiving the process definition information transferred from the transfer device; storage means for storing the process definition information received by the second receiving means; and transmission means for transmitting, when the mail server switching period is completed, the process definition information stored in the storage means to the mail server in the operational state.
According to the present invention, there is provided an exclusive save device for accumulating data transmitted to a communication system and process definition information during period when a device in an operational state and a device in a standby state are switched (device switching period), so that a communication system can be operated without interruption during a device switching period.